the higher on the spectrum the more power you need. 5kw on 590 gives decent coverage, 5kw on 1510 gives you crap.
Yes the signal broadcasts to fish and foreign lands, even if you dropped to 5kw, you still have to protect the other signals on the same freq, adjacent freq's, comply with international agreements, etc.
Where in Greater Boston do you think you are going to put a new AM transmitter site. You might be able to lease space on a tower and diplex (assuming they are not locked into a long term lease at the existing site) but look at the last time an AM tried to do a move and major rebuild... the NIMBY's came out on force and it was in court for a number of years.
real estate is too valuable inside of 495 to support a new antenna farm. look at how many AM's are for sale in other parts of the country, without tower sites, because they sold the land under the sticks for more than the station was worth, and they are selling the license as a shot in the dark rather than letting it go dark.
1510 is screwed